Lamellar Gravity Settlers sludge accumulation in the bottom part. 1

[lcms]

Dear Sirs,

I work at a Rubber Factory Plant that have started up a wastewater treatment plant since february´ 12.

Our worst nightmare (among small others) is the primary physical treatment where we have a rapid on line mixer to mix 200 ppm of alum with our pH adjusted raw wastewater (to pH 8,5) to coagulate the particles which are thereafter floculated with 2.5 ppm of anionic polymer (0.1 % KURITA CP-606 solution of polyacrilamide). The coagulation and floculation under these conditions are just fine, but our lamellar gravity settler bottom is not suitable for complete removal of the sludge formed and the remaining sludge undergoes a anaerobic fermentation that develop some gases (H2S; N2; etc.). This undesired flotation drags some particles to our MBR reactors (2 tanks 350 m3 with 5 400 m2 Microdyn Nadir membranes in each tanks, designed to filtrate 60 m3/h) and we are very concerned that this could clog the extra expensive membranes.

Well, I would really apreciate if one of you have some tips how to improve, or even better, how to design an effective geometry to remove all sludge from the bottom of our gravity settler. I would like to add that my present bottom geometry are two "V" shape tunnels where there is only on bottom perforatede 2 inches pipes in each side and the sludge is remaining between the hole intervals and on the top inclinated (45 degrees) tunnels walls.

If needed I can attached draws or even pictures to improve understanding of our problem.

Thanks in advance,

LCMS.

 

[bimr]

You have not posted the concentration of solids in the wastewater. One would suspect that a high concentration of solids combined with the geometry of the unit is causing the problems.

The 45 degrees tunnel walls is somewhat of a mystery as the manufacturer of the Gravity Settlers should know that the minimum angle of repose for solids will be at least 55 degrees. That means that if the angle is less than 55 degrees, solids will be retained on the side walls. That is also why the plates are at an angle of at least 55 degrees.

I would not recommend thickening in the bottom of the plate settler as the solids loading may be too heavy. I would make the walls drop straight down from the plate settler area instead of at concentrating angles. You can thicken in a separate tank.

The sludge can be removed from the bottom using an arrangement of perforated piping placed off the bottom of the gravity settling tank. The pipes must be regularly spaced, be of a diameter wide enough to be cleaned easily in case of clogging, and the flow velocities should also be high enough to prevent sedimentation. These last two requisites are somewhat contradictory and a compromise is usually reached, say pipes of 3-inch diameter, perforated with holes of 0.75-inch in diameter, 3 feet apart. Flow in individual pipes may be regulated by valves. This configuration is best used after screening.

You may consider a water backflush. The clarifier manufacturers use a short timed periodic water backflush to flush out the collection pipes and holes. It is not clear how big of an area that the sludge collection system covers. The collection system should be subdivided with valving so that you can backflush individual collector pipes.

 

[lcms]

Dear bimr,

Thank you for your valueable tips. I fully agree that the inclination of bottom tunnel wall should be much great. I would have thought something lika 60o degrees (or 55 degrees as you said).

The concentration of sludge is 7.900 mg/l, and the influent must be around 600 to 1,000 mg/l. So, as you can see the sludge is not very concentrated nor stick, but the drainage line is so small and poorly distributed that most of the sludge remains on the settler. I would like to know your opinion about the ideal geometry, since most of good settlers I have seen had inverted pyramides instead of these tunnels.

Regards,

Lcms.

 

[bimr]

One would assume that the manufacturer is attempting to thicken the solids in the bottom by angling in the side walls. There is a limit to what you can do with thickening because the corners have to be at least the 60 degrees slope that you have mentioned. If the corners are 60, then the sidewalls are even steeper than 60.

The main purpose of the unit is clarification not sludge thickening, so there is a limit to what you can do. I would probably have gone with vertical side wall or the inverted pyramid (where the angle of corners set the slopes). Maybe the inverted pyramid would have been the best choice.

I am wondering if you would have been better off with a dissolved air floatation unit instead of a plate settler.

 

[lcms]

Dear bimr,

Thanks for your help. I also have consideres flotation would be the best option for our wastewater, but at this point I will have to consider it as my last alternative since my plant is already runing and a modification of this magnitude would require resources (money; time; manlabour; etc) I do not have anymore. The manufacturer proposed a modification that I am attempted to try ( later today I going to post an attatchment depicting such modification).

My best shoot, on another hand, would be the inverted pyramides with 60 degrees plates, as you also liked.

Anyway, I will wait for our attempts and thereafter I will post the best approach, just to help people to not make mistakes like this.

Best Regards,

LCMS.

 

[bimr]

Note that the angle in the corners has to be at least 60 degrees. The plate angle will be steeper.